1. Field of the Invention
The present invention relates generally to test beds for turbojet engines for aircraft, and more precisely to balances for measuring the thrust of turbojet engines.
2. Summary of the Prior Art
Test beds for turbojet engines consist of a building inside which the turbojet engine to be tested is suspended from a structure for its working tests, while an adjacent control room enables all the tests to be controlled and the results to be evaluated.
Generally, the turbojet engine suspension structure has two functions: firstly it makes it possible to mount the engine inside a building adapted for the tests and acoustically insulated from the control room, and secondly it makes it possible to measure the effective static thrust of the engine.
For this, it is conventional to fit the engine with an adaptor which simulates the attachment arrangement of the turbojet engine so that it is equivalent to that of an actual mounting of the turbojet engine on a support pylon of an aircraft. This adaptor is then fixed under a thrust measuring balance which generally comprises a movable frame, a fixed frame and an interface structure.
Measurement of the thrust is conventional and is accomplished by measuring the longitudinal force generated by the engine when it is operating (thrust) and transmitted to the measuring system through the movable frame and connecting rods for taking the strain.
The movable frame is traditionally fixed to the fixed frame by four vertical flexion strips, and extremely accurate sensors for measuring displacement are disposed between the fixed frame and the movable frame.
The accuracy of the sensors is extremely important because the true thrust of the engine is calculated on the basis of the force which gives rise to the deformations in the various structures through which it passes. The more complex the route, the greater the dispersion and the less accurate the result. To obtain the degree of accuracy of 1 to 2 parts per thousand required for these thrust measurements, it is necessary to provide highly sophisticated mountings as well as extremely accurate fixed and movable frames.
In existing balances, the mountings have four suspension strips and are hyperstatic, creating prestresses in the measuring balance which make it essential to employ even more precise, and therefore more expensive, measuring sensors.
It is an object of the present invention to provide a measuring balance which minimises the aforementioned drawbacks by ensuring a completely isostatic mounting of the engine on its test bed.
A further object of the invention is to overcome the majority of the risks of Prestresses in the measuring balance assembly, which will make it possible to use less sophisticated (and therefore less expensive) measuring sensors than in the past in order to arrive at improved measurement accuracy.